This invention relates to blood pressure measurement involving low frequency vibration detection and phase discrimination. The invention is particularly applicable to blood pressure measurement of chemical warfare garbed casualties. The need to obtain vital signs information in any casualty is indisputable. The following vital signs are typically measured, systolic pressure, mean arterial pressure, diastolic pressure, pulse rate, exhalation rate, and tidal volume exhaled. The present invention is directed to a method of measuring diastolic pressure.
Although the diastolic pressure is a uniquely defined parameter, there is uncertainty with regard to its determination when a sphygmomanometer is used. Over the past 30 years, Phase V, the ceasation of sound and Phase IV, the beginning of muffling, had been used as criteria for determination of diastolic pressure. The true diastolic pressure, as measured with an indwelling transducer, is generally believed to be between the fourth and the fifth phase. The current recommendation of the American Heart Association is to record both Phase IV and V readings. For certain groups of people, including infants, patients with aortic regurgitation, and normal subjects during or following exercise, the muffling of sound is not always easy to discern, and the Korotkoff sounds can be heard all the way to low or even zero cuff pressure. In such subjects, the error in diastolic pressure determination can be quite large.
The diminution of time lag between the arrival of each arterial pressure pulse and the initiation of Korotkoff sound has been found to be a more reliable and accurate criterium for diastolic pressure indexing or measurement.
The present invention broadly measures the time lag between arterial pressure pulse and the next subsequent Korotkoff sound. When the time lag between the pressure pulse and the sound is essentially zero the pressure pulse may be read as the true diastolic pressure.
The method of our invention broadly comprises detecting a first signal corresponding to an arterial pressure pulse; sensing a second subsequent signal corresponding to a Korotkoff sound, determining the time lag between the signals; and providing an output corresponding to diastolic pressure when there is no measurable time lag between the signals.